A color process that applies color correction according to the types of recording media is known (U.S. Pat. No. 6,108,008). Also, a color process used to realize a color appearance of target recording paper on different recording paper sheets is proposed.
FIG. 7 is a block diagram for explaining a color signal process for recording an image of the same color appearance on different recording paper sheets. FIG. 7 shows a color signal process that realizes a color appearance on target recording paper A on recording paper B having a different color reproduction character. In FIG. 7, input color signals R, G, and B are converted into print color signals Rd, Gd, and Bd via a target color converter 5001, mapping section 5002, and media color converter 5003.
More specifically, the target color converter 5001 converts the input color signals into colorimetric signals Lt, At, and Bt of the target recording paper A. The calorimetric signals are color signals that pertain to colorimetric values of colors printed on recording paper, and color signals on a CIEXYZ color space, those on a CIELAB color space, those on a color appearance model color space, and the like are used. A target profile storage section 5004 stores a three-dimensional lookup table (3DLUT) indicating the relationship between the discrete input color signals R, G, and B and colorimetric signals Lt, At, and Bt of the target recording paper A corresponding to the color signals R, G, and B as a target profile. The target color converter 5001 converts color signals by a known interpolation method by utilizing this target profile.
The mapping section 5002 converts the colorimetric signals Lt, At, and Bt into colorimetric signals Ld, Ad, and Bd that can be reproduced on the recording paper B. Since color gamuts that can be reproduced vary depending on the recording paper sheets that are being used, if a given color signal can be reproduced on the recording paper A but cannot be reproduced on the recording paper B, all color signals that can be reproduced on the recording paper A are converted into those which can be reproduced on the recording paper B so as not to change the impression of the color appearance. Basically, a color signal that can be reproduced on the recording paper B is reproduced using nearly the same color without applying any large change, and a color signal in a high saturation area is converted into a color signal whose saturation is compressed while its hue value remains the same. In other words, gamut mapping is performed such that the color gamut of the recording paper A is compressed into that which can be reproduced on the recording paper B. A mapping parameter storage section 5005 stores parameters associated with saturation mapping conditions. The mapping section 5002 converts color signals by known gamut mapping by utilizing these parameters.
The media color converter 5003 converts the colorimetric signals Ld, Ad, and Bd into the printer color signals Rd, Gd, and Bd to be reproduced on the recording paper B. A media profile storage section 5006 stores a 3DLUT indicating the relationship between the discrete printer color signals Rd, Gd, and Bd and calorimetric signals Ld, Ad, and Bd of the recording paper B corresponding to the color signals Rd, Gd, and Bd as a media profile. The media color converter 5003 converts color signals by a known interpolation method by utilizing this media profile.
According to the aforementioned color signal processing method, the recording paper B which has a color reproduction character different from that of the target recording paper A can realize the color reproduction character of the recording paper A.
However, if the gloss character of recording paper varies, good color appearance cannot often be realized by the aforementioned color signal processing method. This results from differences between the illumination upon colorimetry and the geometrical condition of light receiving, and illumination upon image observation and the geometrical condition of the observation direction. Reflected light that comes from a print image and enters the eye of an image observer includes two different components. One component is a diffuse component (to be referred to as “diffuse light” hereinafter) of illumination light, which enters recording paper, and emerges from the recording paper after it has undergone a large number of times of reflection and refraction by paper texture and color materials. The other component is a reflected component (to be referred to as “surface-reflected light” hereinafter) from the relatively smooth surface of recording paper.
A general colorimeter used upon generating a printer profile has a geometric condition called 45-n or -45, which extracts only the aforementioned diffuse light, and does not detect any surface-reflected light. Recording paper which has a small amount of surface-reflected light, recording paper which has a large amount of surface-reflected light but can remove most of it from the field of view upon image observation, and recording paper the surface-reflected light of which enters the eye of an image observer have different appearances of print images even when the colorimetric values are the same. That is, among recording paper sheets having different gloss characters, good color appearance cannot be obtained using the aforementioned color signal processing method.